This invention relates generally to printers and more particularly to a circuit for sequentially enabling electrode energization in an electroerosion printer.
In printers using metallized paper, the metal film is selectively removed by an electrical pulse through an electrode and the metal coating at the point of contact to thereby burn or evaporate the metal coating to leave the contrasting undercoat as a visible mark. Characters are usually formed by the selective energization of a plurality of electrodes as the record medium moves with respect to the print electrodes. In the past, electrode energization has been permitted to occur at any time the electrode received a timed data signal to be recorded. As a result, many electrodes could be fired simultaneously causing a much higher opposing voltage drop across the paper than that encountered with the firing of one or a few electrodes. The current to each electrode or stylus in the large group then is much less than with a small number of electrodes with the result that the intended burned area or dot is only partially formed.
One alternative to this problem is that described in U.S. Pat. No. 3,846,801 in which the plurality of electrodes must each fire individually by a multiplexing circuit. This arrangement has a severe limitation in that the current limiting resistor for controlling the amount of current at the burned area is in the return path from the paper and not in the individual stylus circuit. Thus, the single limiting resistor prevents satisfactory energization of more than one electrode and necessitates a long accumulative firing time to accommodate all electrodes in succession. In addition, there is no alternative to firing a single electrode at any particular instant.
When firing many electrodes at once, a further disadvantage is that of the transmitted electromagnetic radiation, ever present when marking occurs. The concurrent energization of a large number of electrodes results in greater current switching with resultant increase in the transmitted radiation or noise.
It has been found from experience that the current paths through the metal coating of the paper should be maintained as uniformly as possible for each electrode when fired. When the electrodes are arranged to follow one another along the print line and are traversing adjacent to an already recorded area, the firing of leading electrodes can frequently narrow the metal current path for a yet unfired lagging electrode thus creating an increased current path impedance on the paper for the latter electrodes. As a result, incomplete metal removal occurs, providing degraded printing quality.